7Digital client

Implemented the following methods for the version 1.2 of 7digital api:

• artist/browse
• artist/chart
• artist/details
• artist/releases
• artist/search
• artist/toptracks
• artist/similar
• artist/tags
• artist/byTopTags
• track/search
• track/chart
• track/details
• track/preview
• release/bydate
• release/chart
• release/details
• release/recommend
• release/search
• release/tracks
• release/tags
• release/byNewTags
• release/byTopTags
• tag/list

Handle different api error message through meaningfull exceptions.