Add linux support

Author: kylef

Sources/ArgumentParser.swift

@@ -52,10 +52,10 @@ public final class ArgumentParser : ArgumentConvertible, CustomStringConvertible
   /// Initialises the ArgumentParser with an array of arguments
   public init(arguments: [String]) {
     self.arguments = arguments.map { argument in
-      if argument.hasPrefix("-") {
+      if argument.characters.first == "-" {
         let flags = argument[argument.startIndex.successor()..<argument.endIndex]
 
-        if flags.hasPrefix("-") {
+        if flags.characters.first == "-" {
           let option = flags[flags.startIndex.successor()..<flags.endIndex]
           return .Option(option)
         }

Sources/CommandDarwin.swift renamed to Sources/CommandRunner.swift

@@ -1,4 +1,9 @@
-import Darwin.libc
+#if os(Linux)
+  import Glibc
+#else
+  import Darwin.libc
+#endif
+
 
 /// Extensions to CommandType to provide convinience running methods for CLI tools
 extension CommandType {