new api method getTestCaseCustomFieldExecutionValue #12

- examples and tests added for getTestCaseCustomFieldExecutionValue and
getTestCaseCustomFieldDesignValue
getTestCaseCustomFieldDesignValue returns now '' as empty response #12
new api method  getTestCaseCustomFieldTestPlanDesignValue #12
new api method  updateTestCaseCustomFieldDesignValue #12

- examples and tests added for updateTestCaseCustomFieldDesignValue
- example reportTCResult with custom field value added2
new api methods getTest[Suite|Plan]CustomFieldDesignValue #12
new api methods get[ReqSpec|Requirement]CustomFieldDesignValue  #12
cleanup custom field examples #12
api methode reportTCResult - new optional argument 'user' #24
getTestCasesForTestSuite example - wrong argument detail #23
getTestCasesForTestSuite - new optional argument 'getkeywords' #24
getTestCasesForTestSuite - new optional argument 'getkeywords' #23
Merge branch 'generic-api' of https://github.com/lczub/TestLink-API-Python-client into generic-api
prepare pre-release v0.5.0-Beta