Initial implementation of BIP0037.

Implement filteradd, filterclear, filterload, and merkleblock.
Complete BIP0037 support started by dhill.

- Correct MsgFilterLoad max payload
- Enforce max flag bytes per merkle block
- Improve and finish tests to include testing all error paths
- Add fast paths for BloomUpdateType
- Convert all byte fields to use read/writeVarBytes
- Style and consistency updates
- README.md and doc.go updates

Closes #12.