Implemented alert message serialize/deserialize

* Introduced common methods readVarBytes, writeVarBytes.
* Added type Alert which knows how to deserialize
the serialized payload and also serialize itself back.
* Updated MsgAlert BtcEncode/BtcDecode methods to handle the
new Alert.
* Sane limits are placed on variable length fields like SetCancel
and SetSubVer
Cleanup a few nitpicks with recent alert additions.

- Group the new read/writeVarBytes functions together to be consistent
  with the existing code
- Modify the comments on the new read/writeVarBytes to be a little more
  descriptive and consistent with existing code
- Use "test payload" for field name in the tests for the
  read/writeVarBytes functions which is more accurate
- Remove reserved param from NewAlert since there is no point is having
  the caller deal with a reserved param
- Various comment tweaks for clarity and consistency
- Use camel case for fuction params for consistency
- Move the NewAlert and NewAlertFromPayload functions after the receiver
  definitions for code layout consistency

Closes #11.