Reorganize code to match standard go layout.

Get rid of unnecessary bin/ and pkg/ folders. Split source between a
library ("rules") and the binary ("sgrep"). Also locate the repository
at github.com/sgrep/sgrep, which is necessary to include the library
for the binary.
Remove unnecessary configure.sh script.

Using the normal go tools, this script is now unnecessary. A simple
'go get' or 'go install' should suffice.
Fix formatting.

Use gofmt on .go files. Add a hooks folder to hold git hooks that
should be installed for development. Add note about this to new
CONTRIBUTING doc.
Merge pull request #1 from ewencp/go-layout

Rearrange files to match normal go layout