Add `trial` module, calculate days remaining

This new module will contain functions to calculate trial days.

Here we implement a function to calculate the days remaining in a trial
period.

If there are less than 0 days remaining, it returns an error result.

Couldn't use 'error-chain' because it was complaining about `Debug` and
`PartialEq` not being defined. Don't know if it's possible to implement
those traits on 'error-chain''s ErrorKind`, but it was complicated
enough that I ended up not bothering. Went with 'quick_error' instead to
create a custom error type without having to `impl` all the required
traits.

1 files changed, 10 insertions, 0 deletions

diff --git a/src/errors.rs b/src/errors.rs
index beafc9f..5f09c27 100644
--- a/src/errors.rs
+++ b/src/errors.rs
@@ -5,3 +5,13 @@ error_chain! {
         Xdg(xdg::BaseDirectoriesError);
     }
 }
+
+quick_error! {
+    #[derive(Debug, PartialEq)]
+    pub enum DurationError {
+        NegativeDuration(duration: i32) {
+            description("negative duration")
+            display("negative duration: '{}'", duration)
+        }
+    }
+}