wal: OpenFromIndex fails if it cannot find previous index

Example:
We save entry 1, 2, 3 to WAL.
If we try to open 100, it should fail.
etcdserver: use simple PRNG for GenID
wal: add ErrFileNotFound and ErrIndexNotFound
Merge pull request #1092 from jonboulle/dumb_gen_id

GenID: use a fast prnd
Merge pull request #1099 from unihorn/128

wal: OpenFromIndex fails if it cannot find previous index
test: actually test main pkg
etcd: replace -proxy-mode with -proxy
Merge pull request #1106 from bcwaldon/proxy-flag

Replace -proxy-mode=<bool> with -proxy=<enum>
build: ignore calling environment
Procfile: run etcd from bin/etcd
CONTRIBUTING: update link to style guide
proxy: add readonly handler
etcd: add -proxy=readonly
Merge pull request #1112 from bcwaldon/update-link

CONTRIBUTING: update link to style guide
Merge pull request #1110 from bcwaldon/proxy-readonly

Readonly proxy mode
main: add address validation for bind-addr flag
Merge pull request #1113 from jonboulle/unix

main: add address validation
*: s/prevExists/prevExist/
Merge pull request #1117 from bcwaldon/prevExist

*: s/prevExists/prevExist/
Merge pull request #1107 from bcwaldon/Procfile

Isolate ./build from environment