Add initialize optional connection options and block

Improve the Nessus::Client initialize method with optional
connection_options and block that are passed to Faraday::Connection
Delete mjcarey@10.5.5.14
Use yield instead of passing block
Use statement modifier instead of if statement
Give connection_options higher priority
Merge pull request #5 from mwulftange/initialize

Add optional connection options and block to initialize
Fix #3: Migrate #xlst_list into the correct file
Merge pull request #8 from erran/misplaced-file-endpoint

Migrate #xlst_list into the correct file
Canonicalize setting of :json request parameter