Convert targeted sdk flycheck option from list to string

swiftc frontend doesn't support multiple parameters for the `--sdk` key
Use correct function from cl-lib
Correct source argument extraction in flycheck checker
Shortcut linked sources flycheck logic when corresponding variable is nil
Don't active flycheck checker by default

It doesn't work with linked libraries, this means it doesn't work well
with Xcode projects.
Update README
Merge pull request #56 from ap4y/fix_flycheck_issues

Fix flycheck issues
Update copyright
Release 0.3.0
Start work on 0.4
Fix indentation of multiline operator expressions

Expressions with multiple operators over multiple lines got indented one
offset deeper per line break. An additional smie check for being a
sibling should fix this problem.
Merge pull request #61 from ckruse/fix_55

Fix #55: indentation of multiline operator expressions
Add ckruse to the Acknowledgements in README
Change '_' character class to 'symbol constituent'

Resolves 62
Updated syntax: a lot of attributes moved to keywords

- required, dynamic, final, lazy and optional are now keywords
- the assignment attribute no longer exists

Source:
<https://developer.apple.com/library/mac/documentation/swift/conceptual/Swift_Programming_Language/RevisionHistory.html>
Merge pull request #63 from ckruse/updated_syntax

Updated syntax: a lot of attributes moved to keywords
fix: underscores are word characters (Word constituents)

In Swift, underscores are word characters (Word constituents), not
Symbol constituents. This fixes #64
Merge pull request #65 from ckruse/fix_64

fix: underscores are word characters (Word constituents)
Stop overriding user preferences for tab-width.

fixes #68