Enable random access with #size and #seek methods (also #blobs for debugging).
Make #each return enumerator if no block given.
Update .gitignore with all the stuff produced by the C extension build.
Add #pos to retrieve current logical file position, and #pos= (as an alias of seek) to set it.
Don't perform seek if already there.
Update Readme.md for random access changes.
Normalise index to be positive when seeking (but only if valid).
Raise IOError when #seek fails to seek to valid index.
Move find_all_blobs() function to earlier point in file.
Only perform initial scan when necessary.
Merge pull request #1 from martinjos/master

Random access & position tracking