Fixed some typos and bad whitespaces.

This should avoid random unrelated crashes.

* Improved main.py tests

* Small fixes

  - this feature allows conversion of large ranges where
    all entries coalesce to the same representation without bloating
    a dict. For instance, 0x0 to 0xffff all display as "toto" except
    a few odds 0x2 which are displayed as "tutu".
    Before this feature, you needed to write
    enum = {x: 'toto' for x in xrange(0, 0xffff)
    enum[2] = 'tutu'
    XShortEnumField('test', 0, enum)

* Fix IA_PD length error

I tested this and noticed wireshark wasn't parsing the message correctly. Turns out the length is wrong.

* Update regression.uts

Fix test for DHCP6OptIA_PD

* Update length of DHCP6OptIA_PD in test.

This PR makes it possible to check if several flags are all set, by
writting, for example, `if pkt[TCP].flags.SA:`.

It only works for single letter flags (e.g., will work for TCP().flags
but not for IP().flags), and only when getting a value.

Thanks @guedou for suggesting that.

It is now possible to write `pkt[TCP].flags.S = True`.

This makes it possible to write `if pkt[TCP].flags.S: [...]`.