Add possibility to increase maximum count of keys pressed in row

The maximum count controlled by g:hardtime_maxcount variable.
Defaults to 1.
Merge pull request #13 from rasendubi/add_count

Add possibility to increase maximum count of keys pressed in row
Update README.md
Use previous key mappings if they exist
Fix mapping for visual keys
Fix #15 Weird errors when trying to use movement keys

The reason was that HardTimeOn was called every time user enters buffer.
Fix that with calling HardTime only on buffer creation. Also protect
user from calling HardTimeOn several times.
Merge pull request #18 from rasendubi/enable_mappings

Enable previous key mappings
Remove select mode mapping

Change `vnoremap` to `xnoremap`. `vnoremap` maps both select mode and visual mode, `xnoremap` only maps visual mode.