Added jsdocs for Axis.tickFrequency
Response to review comments

Stating 'distance between tick *values*'
Added note that tickFrequency doesn't affect ordinal axes
Added @link to insight.DateFrequency
Merge pull request #227 from niksoper/tickfreq-docs

Added jsdocs for Axis.tickFrequency
Merge pull request #228 from tpkelly/develop

Fix tick colour/font
Merge branch 'upstream' into develop