[stdlib] UnsafeBufferPointer: add ".count"

In answering a forum post I noiced that I wanted this and it was
missing.

Also, extensive comments

Also, rename the length: init parameter to count:.  When writing the
comments for the init function it became painfully clear why we use
"count" is better than "length" especially around pointers and memory:
the former is much less easy to mistake for "length in bytes".  Plus
it's consistent with the new ".count" property

Swift SVN r20609

test/stdlib/NSStringAPI.swift

@@ -321,7 +321,7 @@ NSStringAPIs.test("dataUsingEncoding(_:allowLossyConversion:)") {
     let data = "あいう".dataUsingEncoding(NSUTF8StringEncoding)
     let bytes = Array(
       UnsafeBufferPointer(
-        start: UnsafePointer<UInt8>(data!.bytes), length: data!.length))
+        start: UnsafePointer<UInt8>(data!.bytes), count: data!.length))
     let expectedBytes: [UInt8] = [
       0xe3, 0x81, 0x82, 0xe3, 0x81, 0x84, 0xe3, 0x81, 0x86
     ]