Addition of support for a distance of 1

[mrjamesriley]

For our purposes, it was apparent that the spellchecker was too 'generous' in the suggestions offered for words - due to the edit distance of two. This meant that words which we wouldn't deem to be typos, frequently being identified as such - or we'd have candidates offered which were too far 'removed' from what we'd expected. It also happens to be that the performance was a fair bit slower than the real-time use we were aiming for.

When set to an edit distance of 1, we get the performance and tighter 'accuracy' that works well for us. Thus this pull request is to allow for the setting of the distance during the SpellChecker initialisation, with the default set to 2 for backwords compatability.

A quick benchmark reveals the increased performance of the edit distance of 1, where appropriate of course:

word: 'mrjamesriley'

Looking for correction with distance of two (5 times):
15.7 seconds

Looking for correction with distance of one (5 times):
0.01 seconds

[coveralls]

Coverage increased (+0.01%) to 98.425% when pulling db4edaf on mrjamesriley:master into 2bdb305 on barrust:master.