fix SPDY headers so dependent projects can stop using pch files

[johnkdoe]

when attempting to build our project that depends upon
CocoaSPDY without precompiled headers, we found a
couple of errors:

• SPDYError.h refers to NSString
  • solution: import Foundation.h
• SPDYError.h has externs that could be in C, C++ or Objective-C code
  • solution: use FOUNDATION_IMPORT, found in Foundation.h
• SPDYLogger.h refers to NSString
  • solution: import Foundation.h

[coveralls]

Coverage remained the same when pulling daf4d98 on johnkdoe:import-cleanup into 4bd59a8 on twitter:develop.

[goaway]

Thanks!