etcdhttp: perform validation of query parameters

Add basic input validation of all query parameters supported by
serveKeys. Also restructures etcdhttp a bit to better facilitate
testing.

Test coverage is slightly improved.
etcdhttp: add tests for ParseBool/ParseUint64 helpers
etcdserver: address style comments
etcdserver: switch to using etcd.Error
wal: add a doc file
etcdserver: restructure to hopefully simplify

Obviate parseBool helper, define emptyReq locally to parseRequest,
have writeEvent return an error which gets logged
test: test wait pkg
test: sort the pkgs
wal: add more explanation to wal filename format
wal: wal.OpenFromIndex -> wal.OpenAtIndex

The first entry read from the wal should be the index provided. This
name makes that more clear.
Merge pull request #1056 from coreos/test

test: test wait pkg
raft: log comment grammar fix
Merge pull request #1058 from philips/grammar-fix

raft: log comment grammar fix
etcdhttp: use Form values over query parameters
simplify
Merge pull request #1011 from jonboulle/clean_http

Validate HTTP API user input
etcdserver/etcdhttp: split out peers and add tests
raft: test node step
Merge pull request #1054 from philips/add-wal-doc

wal: add a doc file
etcdserver: switch to proper int64Slice for sorting