It's useful to be able to dump regions of memory a la `xxd'. Add some
machinery to parser_util and RamDump to do this.

Example:

    $ ramparse.py -a . -o parsed --shell
    >>> dump.hexdump(0, 100, virtual=False)
    00000000: fffe ffde 0200 0000 ffff b7bf adf5 a5ff  ................
    00000010: e7bf edfe c6fe edf6 fefa fffe befa fefe  ................
    00000020: bfbe ffef aebd a5ef ffff 7f7e fefe 5afb  ...........~..Z.
    00000030: 1b00 2000 3300 4300 1c00 2100 3400 4800  .. .3.C...!.4.H.
    00000040: ffff ff7f fafe ffdb effd a5af adf5 a5bf  ................
    00000050: 4444 4444 4444 4444 4444 4444 4444 4444  DDDDDDDDDDDDDDDD
    00000060: 5fff ffff                                _...
    >>> dump.hexdump(0xc0000000, 100, virtual=True)
    c0000000: fffe ffde 0200 0000 ffff b7bf adf5 a5ff  ................
    c0000010: e7bf edfe c6fe edf6 fefa fffe befa fefe  ................
    c0000020: bfbe ffef aebd a5ef ffff 7f7e fefe 5afb  ...........~..Z.
    c0000030: 1b00 2000 3300 4300 1c00 2100 3400 4800  .. .3.C...!.4.H.
    c0000040: ffff ff7f fafe ffdb effd a5af adf5 a5bf  ................
    c0000050: 4444 4444 4444 4444 4444 4444 4444 4444  DDDDDDDDDDDDDDDD
    c0000060: 5fff ffff                                _...
    >>> dump.hexdump(dump.addr_lookup('linux_banner'), 144, virtual=True)
    c0b0006a: 4c69 6e75 7820 7665 7273 696f 6e20 332e  Linux version 3.
    c0b0007a: 3130 2e32 382d 6765 3232 3362 6632 3830  10.28-ge223bf280
    c0b0008a: 662d 6469 7274 7920 286d 6974 6368 656c  f-dirty (mitchel
    c0b0009a: 6840 6d69 7463 6865 6c68 2d6c 696e 7578  h@mitchelh-linux
    c0b000aa: 2920 2867 6363 2076 6572 7369 6f6e 2034  ) (gcc version 4
    c0b000ba: 2e37 2028 4743 4329 2029 2023 3720 534d  .7 (GCC) ) #7 SM
    c0b000ca: 5020 5052 4545 4d50 5420 5475 6520 4170  P PREEMPT Tue Ap
    c0b000da: 7220 3820 3134 3a32 303a 3132 2050 4454  r 8 14:20:12 PDT
    c0b000ea: 2032 3031 340a 007c 2f2d 5c00 0000 0000   2014..|/-\.....

Change-Id: Iecaf80f72845f052085a60eadedd2bb24743224c