CHANGE: make the std feature default (breaks no_std)

Software that uses this crate in a no_std environment will now have to clear the `std` flag
manually.

Rationale:

Creating documentation and testing things that require `std` would require a lot conditional
compilation tricks. Things would easily go under the Radar when buildiong doc and running
tests with `no_std` being the default.

Most users likely expect the `std` feature to be enabled by default.

Still if it this is a problem we can keep `no_std` being the default with some effort.

Author: cehteh

Diff for: Cargo.toml

@@ -12,7 +12,7 @@ license = "MIT"
 readme = "README.md"
 
 [features]
-default = ["atomic_append"]
-atomic_append = []
+default = ["std", "atomic_append"]
 std = []
+atomic_append = []
 

Diff for: README.md

@@ -8,4 +8,8 @@ If you use this without creating a weak ptr, it is safe. It is unsafe to create
 
 * `std`  
   Enables API's that requires stdlib features. Provides more compatibility to `Vec`.
-  This feature is not enabled by default.
+  This feature is enabled by default.
+* `atomic_append`  
+  Enables the `atomic_push` API's that allows extending a `HeaderVec` with interior
+  mutability from a single thread by a immutable handle.
+  This feature is enabled by default.